1. Field of the Invention
The present invention relates to a detector for detecting voltage breakdowns on the high-voltage side of an electric precipitator, the voltage of which is supplied from an a-c network through a thyristor control circuit, a high-voltage transformer, and a rectifier.
2. Description of the Prior Art
Detectors of the above-described type are known in the art (see, for example, Siemens-Zeitschrift (1971), pp. 567-572), and are used in electric precipitators. In such precipitators, the applied d-c voltage is slowly and gradually increased up to a voltage level at which a breakdown in the precipitator gap occurs. When such a voltage breakdown occurs, the voltage at the precipitator decreases immediately. After a voltage breakdown, the drive voltage for the single-phase thyristor control circuit on the a-c side must be quickly reduced. If it is not reduced, a current of large magnitude would flow through the ionized gap of the precipitator during the next half-wave of the precipitator voltage, which is undesirable. After the drive voltage is reduced, the precipitator voltage is slowly increased back up to its breakdown level.
The slope of the decrease of the precipitator voltage measured during the breakdown of the voltage at the precipitator can be used as the criterion for determining a precipitator breakdown. The voltage slopes which are present during normal operation of the precipitator due to the variation of the precipitator voltage must, however, be substantially lower in order to do so. It should be noted that the high-voltage rectifier of such a detector generally comprises a two-pulse rectifier, and that the electric precipitator is a considerable ohmic load due to its corona discharge. As a result, the precipitator gap voltages exhibit corresponding harmonics, and the waveform shape of the voltage will depend upon the precipitator design, the matter which is to be precipitated, and the size of the precipitator installation, particularly during operation at a low breakdown voltage. For these reasons, measurement of the voltage slopes during normal precipitator operation and during a voltage breakdown must be carried out in different precipitator installations and under different operating conditions in order to obtain accurate criteria for detecting a breakdown by measurement of the voltage slopes.